The IEEE 802.3af Ethernet standard, also known as the Power over Ethernet (PoE) standard, describes a system to safely pass electrical power, along with data, on Ethernet cabling in a powered network. In addition to carrying data as in a traditional network, powered networks also provide DC power to one or more powered devices (PDs). By connecting a PD to a powered network, such device does not require additional AC wiring or an external power source.
In a powered network, power sourcing equipment (PSE) is configured as the source of DC power. Such PSEs usually reside at distribution points in the powered network, such as an Ethernet hub, switch, router, or other network equipment. A PSE can be configured in a variety of standard as well as non-standard modes for delivering various levels or classes of power. Each of the IEEE 802.3af and the more recent IEEE 802.3 at (also known as PoE+) standards contains specifications for different levels and configuration details. One part of each standard provides each PSE a mechanism to detect the presence of a PD at a network port, such as by sensing the resistance of a PD. For instance, the PSE can detect whether a PD is attached as part of a start-up procedure before applying power.
The PoE standards also specify two general means for detecting disconnection of a PD, a DC disconnect method and an AC disconnect method. Regardless of the disconnect method, the PoE standard requires that the PSE shuts down power to a disconnected port within a predetermined time. The DC disconnect method involves monitoring a minimum DC current draw of about 5 mA to about 10 mA. The AC disconnect method that involves monitoring the AC impedance of network ports.
Existing approaches for detecting disconnection tend to result in significant power dissipation and/or are expensive to implement.